Half life

					HALF LIFE
MEASURING RADIOACTIVITY
• All unstable radioactive atoms
  will eventually lose their
  radioactivity – they decay.
• We can measure how quickly a
  radioactive atom decays by
  measuring the half-life.
• This is the average time taken
  for half the atoms to decay
  (lose their radioactivity)
                   HALF-LIFE
• Plot the following data.
COUNT
RATE
(bequerels)
            95      73   55   42    32    23   18
Time           0    10   20   30    40    50   60
(secs)

  Count rate




                                   Time
• Complete the following

• The time at 90 counts is …………….

• The time at 45 counts is …………….

• 45 is half of 90 so the time it takes for the
  counts to drop by half is ________

• Pick two more pairs of counts where one
  is half the other and work out the half-life.
    HALF-LIFE
R
a
d
i
                   For this substance
o
a                  Half life – the time (37
c                  hours) for the
t                  radioactivity to drop by
i                  half ( 16 to 8)
v
i
t
y

    Time (hours)
ANOTHER WAY TO CALCULATE HALF
            LIFE

• Instead of drawing the graph we can
  calculate the half life from the data
  given.
• EXAMPLE
• A radioactive isotope has a count rate
  of 400 becquerels. 12 days later it is
  50. What is the half life?
        After 1           After 2            After 3

        half life         half lives         half lives
• 400               200                100                50

• 3 halflives have passed for the counts
  to fall from 400 to 50
• This took 12 days.
• So 12 (days) / 3 (halflives) = 4 days.
• The half life is 4 days
• Each Radioactive
  substance will have it’s
  own half life.
• Some radioactive isotopes
  have very long half life’s
  (many thousands of years)
  and others have very short
  half life’s (seconds)

				
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posted:4/5/2010
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